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Old 12-20-2020, 04:00 PM   #1
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Furnace output and limiting switch?

My furnace started making a horrible squeal noise so I have a new motor on the way. It's time to address another issue: the furnace short cycles. This to me indicates that the combustion compartment isn't getting enough air through it.....

The vent that exits the side of the air handler blows like a blow dryer.... The other 4 vents? They blow like an asthmatic geriatric. Is that normal?

If so, how can I get the unit to flow more so that it doesn't cut out after 3-5m of heat?
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Old 12-20-2020, 06:42 PM   #2
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The motor noise may also be an indication of poor motor speed , you may have enough air flow to keep the " sail switch " from shutting down ,or not starting the ignition cycle , but not enough to keep the over temp sensor from shutting down after a short run time .

When the motor arrives don't just put it in ; pull the whole furnace out for a complete cleaning , combustion chamber/heat exchanger, inside and out .
While helping a friend with a short cycle issue , we got about 6 lbs of mud dauber nest out of the combustion chamber.

EDIT : Take your furnace info( make and model ) and look for a " Tune up kit " online .
Kit for mine included , sail switch, temp sensor, burner assembly and gasket .
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Old 12-20-2020, 07:32 PM   #3
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I’m curious about the furnace also. My vents are weak. I assume the furnace pulls out the side of the RV, and you don’t need to mess with the fridge to get it out? I put in a residential fridge and don’t want to pull that thing out again.

I know mine is an Atwood 8940-iii-DCLP
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Old 12-20-2020, 07:38 PM   #4
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Different furnaces , different amount of work to remove .

My Suburban , 20 mins to put the furnace on the bench.
Remove outside panel , disconnect wiring, and propane , one screw and the furnace is in hand.
Red box is the heat exchanger , silver plate in the box is the burner access.
Red circle is the bracket for the mount screw.

EDIT : All instructions were in the install manual .
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Old 12-20-2020, 07:44 PM   #5
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I have a suburban furnace. It is mounted under the refrigerator. When I first bought the rig I had to redo (best I could with the cramped quarters) the flexible ducting and connections. They did a poor job of connecting and sealing the connectors to the furnace. The also had too long of ducting to some of the vents causing restrictions, I cut some off and made the runs as straight as possible. Seemed to work OK after I worked on it.
Since then I replaced the Norcold with a Samsung, had to lower to where there is not enough room to work on the furnace without removing the fridge but when I did have the furnace out I cleaned it up good with compressed air and made sure everything was working as it should including the ducting.
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Old 12-20-2020, 08:25 PM   #6
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Quote:
Originally Posted by JerryLook View Post
I know mine is an Atwood 8940-iii-DCLP
This is the same model that I have. It does heat up the rig quite well but takes a long time when only cycling for 3-5m.
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Old 12-20-2020, 09:36 PM   #7
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When I put the residential fridge in mine I had to lower it down to fit also. I looked up the install specs for this furnace and it said I need at least a 1/2” of clearance. I gave it an inch between the furnace and bottom of the fridge

I think I’d have to pull my fridge to get my furnace out now too. I shouldn’t taken it out and cleaned it when the fridge was out, but I was doing the install in the summer (in Florida), and I didn’t even want to look at that furnace I was so tired of the heat

Frank, I found a tune up kit for 53$ dollars here

https://pdxrvwholesale.com/products/...ii-tune-up-kit

I might get one and pull the fridge to make the furnace is 100%. Mine heats the coach fine too, but I’d like to be sure it’s running well before I use it too much. We usually just heat with a Vornado right now.
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Old 12-20-2020, 09:49 PM   #8
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If installed with an exterior panel components accessible

https://pdxrvwholesale.com/products/...ii-tune-up-kit
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Old 12-27-2020, 05:28 PM   #9
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Replaced the blower motor and did some experimenting...... It no longer short cycles if I uncover some of the knockouts on the output ducting. This tells me that the short cycles are due to insufficient airflow over the heat exchanger.

I'm not sure how to "fix" this. Removing some of the block-off plugs isn't a real fix - in fact, it causes less heat to be directed to the cab or bedroom. I'd prefer more heat to the cab and bedroom! I could add a 12v auxiliary blower to the top of the heat exchanger but otherwise I'm not sure how to get more hot air out of there?
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Old 12-28-2020, 11:14 AM   #10
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The ducting on my furnace is all routed under the false floors of the base cabinets and/or into the basement. I had a couple that had way too much ducting that they crammed into the space causing quite a bit of restriction. I cut them off as short as possible and reattached to the vent. It helped balance the amount of air coming out along with more even temperatures. It took some time to straighten it all out but worth a look.
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Old 12-28-2020, 12:02 PM   #11
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Quote:
Originally Posted by Fffrank View Post
Replaced the blower motor and did some experimenting...... It no longer short cycles if I uncover some of the knockouts on the output ducting. This tells me that the short cycles are due to insufficient airflow over the heat exchanger.

I'm not sure how to "fix" this. Removing some of the block-off plugs isn't a real fix - in fact, it causes less heat to be directed to the cab or bedroom. I'd prefer more heat to the cab and bedroom! I could add a 12v auxiliary blower to the top of the heat exchanger but otherwise I'm not sure how to get more hot air out of there?
8940-III Installation Manual
Pg 4 outlines the Required Ducting
http://techsupport.pdxrvwholesale.co...ion-Manual.pdf
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Old 12-28-2020, 02:54 PM   #12
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Make sure you inspect your burner for any cracks. It could be overheating do to that and shutting off as it should. If you think it is blocked ducts, just leave the duct work off temporarily and do not mount the furnace all the way in if needed. If it short cycles, you have other issues. The motor dragging may have allowed excessive heat for too many cycles and damaged the overtemp sensor or as mentioned, the burner. Look closely.
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Old 12-30-2020, 12:17 AM   #13
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Opening one more of the knock-outs and blowing a massive amount of heated air out from below the fridge ends the short cycling.

The only thing I can figure is that the floor ducts are obstructed..... I get almost no air flow out of them. But how is it possible I have such serious obstructions both front and back in the ducting (which all appears to run under the floor?)

Because all of the heat comes out in the kitchen area, the unit is now cycling correctly but the thermostat is awful. It wants to maintain a 1 degree sweet spot. So when it's cold (like it is now) it's coming on for 2-3 minutes and then back off for 1-2 minutes. Annoying!
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Old 01-03-2021, 03:11 PM   #14
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I hate this furnace.

Twice now while driving it has stuck "on" and heated up the coach to 80F. Turning it off and on via the switch doesn't fix it but cycling it through the modes does seem to. Just annoying to have to pull over to do this only to have it happen again.

Anyone have any ideas?
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